Lanthanide-based catalyst systems that comprise a lanthanide compound, an alkylating agent, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents. Nevertheless, when applied to bulk polymerization of conjugated dienes, lanthanide-based catalyst systems generally provide cis-1,4-polydienes.
Polydienes may be produced by solution polymerization, wherein conjugated diene monomer is polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and products, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent.
For example, conjugated diene monomer has been polymerized in solution using metallocene complexes and/or half-metallocene complexes based upon lanthanoid elements, scandium and yttrium. These catalyst complexes have been used in conjunction with aluminoxanes, organic aluminum compounds and ionic compounds to produce polydienes having a high cis-1,4-content.
Nevertheless, the presence of solvent presents a number of difficulties. The solvent must be separated from the polymer and then recycled for reuse or otherwise disposed of as waste. The cost of recovering and recycling the solvent adds greatly to the cost of the polymer being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
Polydienes may also be produced by bulk polymerization (also called mass polymerization), wherein conjugated diene monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced.
Catalyst technology, particularly as it pertains to the polymer synthesis, can often be unpredictable, and the transfer of technology from one system to another, or the altering of certain variables, can offer technological difficulties. Thus, while bulk polymerization systems offer a number of advantages, the transfer of known solution technology to bulk systems can be unpredictable and filled with technological challenges.